Lidar has been a research tool for over twenty years. However even though there are significant potential applications, routine operational use of lidar, in the mode common for radar, has not happened. There are three primary limitations which have held back the use of current conventional lidar systems. The first factor is the lack of eye safety of pulsed laser transmitters for ground personnel and aircraft operations. Safety considerations oblige that current systems require constant supervision whenever non eye safe laser beams are in use. A second limitation is the cost, size and complexity of the lidar systems that are now employed for atmospheric research. Third, although the technology is rapidly advancing, the lack of reliability of conventional pulsed lasers has been a problems for routine lidar use. The effect of the second and third, and also first, factors given above is that current lidar use is very personnel intensive. For routine or full time lidar observations for applied or scientific applications eye safe, turn key, autonomous systems are what are needed.
Two previous types of laser radar systems in the prior art have the same applications as the present invention. Laser diode ceilometers, which are commercially available and widely used are in fact solid state, eye safe, autonomous laser radar instruments. However their performance is rather limited. Cloud detection is limited to 3 to 7 km, and the ceilometers do not reliably detect cirrus clouds. A study has been made of the application of diode ceilometers to profile boundary layer aerosols. The results were generally unfavorable. Whether the performance of current diode ceilometers may be significantly enhanced over current instruments is doubtful.
A high PRF lidar with photon counting data acquisition is the University of Wisconsin's High Spectral Resolution Lidar (HSRL). In concept, the HSRL is similar to the present invention. However, the HSRL was intended for a specific complex measurement which involves spectral separation . of the molecular and aerosol backscatter return, and the original HSRL was based on a CuCL laser and employs complex etalon filters and PMT detectors. The emphasis of the present invention is as a simple general purpose elastics scattering lidar for cloud and aerosol applications.
Another prior art approach for eye safe lidar is to operate at near infrared wavelengths which are beyond the transmission range of the eye's cornea. In a recent aerosol backscatter experiment, a lidar system was operated at the eye safe wavelength of 1.54 .mu.m. However, at present, incoherent near infrared lidars are severely limited in performance by the noise level of available detectors. A large, high power lidar is required to obtain aerosol measurements. Near infrared lidars which employ coherent signal detection have much potential but are complex and will likely remain sophisticated and expensive instruments.
Recent advances in technology make possible what I believe can be considered a new type of laser radar system which is eye safe, simple, low cost and capable of full time unattended aerosol and molecular scattering observations. Conventional visible wavelength lidar systems which have been employed for most published lidar work to date make use of 0.1 to 1.0 Joule class lasers and operate at a pulse repetition rate of up to several tens of Hertz. The basic design of the lidar that I will describe below is a system with micro Joule level pulse energies and pulse repetition rates of several thousand Hertz and which also employs efficient photon counting signal detection and acquisition. The low pulse energy permits transmitted beam energy densities that are within eye viewing safety standards. With micro Joule pulse energies the return signal levels for aerosol and molecular observations are within the range where quantum noise limited photon counting detection is necessary. Technologies which now make my lidar practical are small diode pumped Nd:YAG and Nd:YLF lasers, solid state Geiger Avalanche Photo Diode photon counting detectors and single card, low cost multi-channel scalar signal acquisition. Since the components are all solid state, high reliability is achieved.